Operating a multiple-access optical network

ABSTRACT

A method of operating a multiple-access network. The network has a number of communication stations having receiving and transmitting portions (1, 2: 3, 4: 5, 6). The stations are physically coupled together by optical waveguides and arranged such that each transmitting portion (1, 3, 5) can transmit signals to one or more receiving portions (2, 4, 6) but cannot transmit signals to the receiving portion of the same station. One method comprises enabling two stations to communicate with each other by transmitting information simultaneously in the same channel. Another method comprises enabling two stations to communicate with each other by causing the transmitting portion of one station to transmit an interfering signal in the same channel as the information transmitted by the transmitting portion of the other station, the interfering signal being such that at other receiving portions the information and interfering signals are received together.

A multiple-access optical network has a plurality of communicationstations, the stations being physically coupled together by opticalwaveguides (usually optical fibres) and arranged such that ecah stationcan transmit signals to one or more of the other stations. Suchcommunication networks are referred to as multiple-access (MA) networkssince any station can receive signals from a number of the otherstations.

Recently proposed multiple-access networks have been based on opticalfibre links which offer a relatively secure, closed, transmissionmedium. However, one of the most attractive signal distribution schemesfrom a network insertion loss and reliability point of view, i.e. thetransmission-star coupler, has a disadvantage of distributinginformation transmitted from one station to a number of the otherstations. This information could be protected using data encryptiontechniques but this is costly.

In accordance with the present invention, a method of operating amultiple-access optical network having a plurality of communicationstations, the stations being physically coupled together by opticalwaveguides and arranged such that each station can transmit signals toone or more of the other stations but cannot receive signals it hasitself transmitted, comprises causing one station to transmitinformation over a selected channel to a receiving station an causingthe receiving station to transmit simultaneously a signal over the samechannel.

This invention finds use, for example, in the type of passivemultiple-access network which is described in more detail in publishedGB patent application No. 2172165A, "Optical Signal Power Divider", ofFaulkner and Healy, published on Sept. 10, 1986 and issuing on Mar. 1,1989, in the name of British Telecommunications plc. The power dividingnetwork described in that patent application sets out to reduce thenumber of communications channels required in a passive network by usinga topology in which a transmitting station is unable to receive its owntransmitted signal.

The present invention aims to provide a method of operating networkswith passive network topologies in which a transmitting station cannotreceive its own transmitted signal to provide a natural degree of datasecurity.

In some cases, both stations may transmit information to each other overthe same channel.

Thus, the method according to the present invention of causing onestation to transmit information over a selected channel to a receivingstation and causing the receiving station to transmit simultaneously asignal over the same channel, may comprise operating the network infull-duplex operation over a single transmission channel.

In this case, stations not associated with the two communicationstations will receive both transmissions simultaneously and it will bevery difficult for those stations to decipher the two simultaneoussignals in such a busy channel, giving a degree of natural datasecurity. The two communication stations, on the other hand, can use thesame transmission channel without causing mutual interference.

In other cases, the method comprises causing the station which is toreceive information to transmit an interfering signal in the samechannel as the information transmitted by the other station, theinterfering signal being received at each of the other stations whichalso receives the information.

This second arrangement provides a more secure transmission system thanin the first cases mentioned above allowing each receiving station toprotect its own incoming information by effectively blocking all otherstations from receiving information in that channel.

Conveniently, the station transmitting information may monitor its ownreceiving channel to confirm that the interfering signal is presentbefore information is transmitted.

The channels may be based on any multiple-accessing technique such aswavelength, frequency, time, or code division.

The physical connections between stations may be provided by opticalfibres, typically monomode optical fibres.

The multiple-access network may include an optical signal divider inaccordance with any of the examples describe in the afore-mentionedpublished GB patent application 2172165A, the divider comprising X inputports and N output ports; and coupling means for optically coupling eachinput port with N-Z output ports, where N-Z>1 and Z>0, whereby in usethe signal power in each input port is divided between the respectiveN-Z output ports.

An example of a method of operating a multiple-access optical networkaccording to the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a transfer matrix of a multiple-access network suitable foruse with the invention; and

FIG. 2 is a block diagram of a multiple-access network.

FIG. 1 illustrates a transfer matrix for a power divider for use in amultiple-access optical network. This illustrates that each one of thegroup input ports P₁, P₃, P₅ is connected to two ports of the group ofoutput ports P₂, P₄, P₆ with the input power to each input port equallydivided between each of the respective two output ports. For example,the input port P₁ is connected to the output ports P₄, P₆. It should benoted that the input port P₁ is not connected with the output port P₂.This feature leads to a number of advantages including a fifty per centsaving in the number of transmission channels if full-duplex operationis required. In addition, collision detection network access proceduresmay be greatly simplified since a collision would be indicated by thesimultaneous reception of signals in a busy station's transmissionchannel. The looped signal paths which can occur in interconnectedtransmissive style networks can also be prevented with these newnetworks if multiple routes between transmissive star couplers areavoided.

An example of a multiple-access network using an optical power dividerbased on the FIG. 1 transfer matrix is shown in FIG. 2. The networkshown in FIG. 2 has three transmitting/receiving stations comprisingrespective pairs of transmitting and receiving portions 1, 2; 3, 4; and5, 6. For clarity, the different portions 1-6 have been separated in thedrawing although in practice the corresponding transmitting andreceiving portions will be physically associated and may be widelyspaced from other stations.

Each transmitting portion 1, 3, 5 feeds signals to an input port of arespective coupler 7, 8, 9. The output ports of each coupler areconnected to respective pairs of Y couplers 10, 11, 12 whose outputs areconnected to respective receivers 4, 6, 2. For example, the output portsof the coupler 7 are connected to input ports of the couplers 10, 11. Itwill thus be seen that the transmitting portion 1 is physicallyincapable of transmitting signals to the receiving portion 2.

Typically, the singals transmitted will be optical signals in which casethe connections between couplers may be provided by optical fibres.

Consider an example where the station having transmitting/receivingportions 1, 2 wishes to communicate with the station havingtransmitting/receiving portions 3, 4. Initially, a suitable channel (A)is chosen which may be based on any multiple-access technique, mentionedabove, for example, a TDMA system may be used.

As previously explained, if the transmitting portion 1 transmitsinformation on channel A this will be received not only by the receivingportion 4 but also by the receiving portion 6. If simultaneouscommunication between stations is required then the same channel A canbe used by both transmitting portions 1, 3 since the respectivereceiving portions 2, 4 will not receive information from the associatedtransmitting portion. However, the receiving portion 6 will receiveinformation in the same channel from both transmitting portions 1, 3.This will make it very difficult for the receiving portion 6 todetermine the nature of the information.

Alternatively, where non-simultaneous communication is required adifferent method may be used. If the transmitting portion 1 is totransmit information to the receiving portion 4 on channel A, thetransmitting portion 3 transmits an interfering signal on the samechannel, channel A, as the information from the transmitting portion 1.This interfering signal will be received by the receiving portions 2, 6but not by the receiving portion 4. Thus the receiving portion 4 is freeto receive the information transmitted by the transmitting portion 1.The interfering signal is chosen to have the maximum interferenceeffect. If a TDMA system using pulse-position-modulation is used, theinterfering signal could be provided by a random pulse position signal.

In some cases, the transmitting portion 1 may delay sending informationuntil its associated receiving portion 2 has detected the existence ofthe interfering signal on channel A.

I claim:
 1. A method of operating a multiple-access optical networkhaving at least three communication stations, the stations beingphysically coupled together by optical waveguides arranged such thateach station can transmit signals to plural of the other stations overinformation transfer channels based on a multiple-access technique butcannot receive signals it has itself transmitted, the methodcomprising:causing one station to transmit information using a selectedinformation transfer channel to a receiving station; and causing thereceiving station to transmit simultaneously a signal using the sameinformation transfer channel whereby other stations also receiving onthe same information transfer channel receive a confusing mixture oftransmissions from said one station and from said receiving station. 2.A method according to claim 1, further comprising communicating betweensaid one station and said receiving station by transmitting informationsimultaneously therebetween using the same information transfer channel.3. A method according to claim 1, further comprising causing the stationwhich is to receive information to transmit an interfering signal in thesame channel as the information transmitted thereto by the said onestation, the interfering signal being received at each of the otherstations which also receives the information.
 4. A method according toany one of claims 1 to 3, wherein the channels are based on one of thewavelength, frequency, time, or code division system.
 5. A methodaccording to any one of the preceding claims 1, 2 or 3 wherein thenetwork includes an optical signal power divider comprising X inputports and N output ports; and coupling means for optically coupling eachinput port with N-Z output ports, where N-Z>1 and Z>0, whereby in usethe signal power in each input port is divided between the respectiveN-Z output ports.
 6. A method according to any of the preceding claims1, 2 or 3 of operating a passive multiple-access optical network,
 7. Amethod according to claim 6 of operating a transmissive star network. 8.A method for enhancing communication security between a pair ofcommunication stations (I, II) in a multiple access communicationnetwork wherein at least three communication stations (I, II, III), eachhaving a signal transmitter and a signal receiver, are physically linkedby fixed communication paths which permit signal transmission from eachstation to plural other stations over selectable signal transmissionchannels but which stations can not receive their own signaltransmissions via said fixed communication paths, said method comprisingthe steps of:transmitting first signals from a first station (I) to asecond station (II) and to at least one other station (III) over saidfixed paths using a selected signal transmission channel (A);simultaneously transmitting second signals from said second station (II)to said first station (I) and to said at least one other station (III)over said fixed paths also using the same said selected signaltransmission channel (A) thereby causing said at least one other station(III) to receive a confusing mixture of said first and second signalswhile the first station receives only said second signals and while saidsecond station receives only said first signals.
 9. A multiple accesscommunication network comprising:at least three stations each includinga receiver and an associated transmitter, each said receiver beingphysically connected to the transmitter of every station in the networkexcept that of its own associated transmitter; each said associatedtransmitter including means for transmitting first signals to anotherstation receiver on a selected channel; and each said receiver includingmeans for receiving said first signals from another station transmitterand for causing its own said associated transmitter to transmit,simultaneously to the said another station which transmitted the firstsignals, second signals on said selected channel, whereby the remainingstations all receive a confusing mixture of said first and secondsignals on said selected channel.
 10. A network as in claim 9,comprising an optical network wherein said stations are connected byoptical waveguides.
 11. A network as in claim 10, wherein said channelis based on a wavelength or frequency division multiplex system.
 12. Anetwork as in claim 10, wherein said channel is based on a time divisionmultiplex system.
 13. A network as in claim 10, wherein said channel isbased on a code division multiplex system.